Making Strides in Tissue Engineering

Career-ending Achilles tendon tears in professional athletes. A decline in an aging population’s quality of life due to injured rotator cuffs. Outdoors enthusiasts made immobile because of tendon tears in their knees.

In the near future, the debilitating nature of these injuries could be a thing of the past, as a team of faculty and students in the University of Idaho’s Department of Biological Engineering is focusing on revolutionary research to engineer regenerative tendon tissue.

Because tendons undergo repetitive motions and sustain such large mechanical loads, they’re prone to injury, said Nathan Schiele, assistant professor of biological engineering in U of I’s College of Engineering. Once injured, treatment involves suturing the tendon through surgery, followed by rehabilitative therapy. Re-rupture is a common occurrence, and patients rarely regain the mechanical strength they once had.

For Schiele, that prognosis is daunting. Having suffered his own share of tendon pain, the second-year professor doesn’t take for granted a healthy musculoskeletal system necessary for an active lifestyle.

“I like to hike and ski and bike, so it’s partially selfish to work on tendons because they’re so crucial to an active lifestyle,” Schiele said. “I want to keep doing these activities, even as I get older, so having an alternative treatment option besides sutures seems like a good idea to me.”

The main focus of his research is to better understand the mechanisms behind successfully engineering tendon tissue through stem cell differentiation.

One of the challenges with stem cells, Schiele said, is that once they’re harvested from a person’s bone marrow or fat tissue and injected into the injury site for regeneration, they remain undifferentiated. This means that the cells, which have the potential to replicate various cell types in the body, can travel down any number of lineage tracks — bone, cartilage, muscle, fat or tendon, posing great risk to the patient.

Schiele and his student research team are trying to ensure proper differentiation of functional tendon tissue in the lab. Such a discovery could eventually allow doctors the ability to extract stem cells from a patient, differentiate them toward tendon cells in the lab, place them on an engineered tissue scaffold that mimics the mechanical strength of tendon, and suture them back into the patient.

“For people who have had major trauma, like an Achilles tendon rupture, we aim to replace or augment that injured tissue with a mechanically functional tendon replacement with cells that act like tendon cells,” Schiele said.

A number of factors exist that can ultimately push a stem cell toward a desired lineage — the shape of the cell, the biochemical environment or growth proteins the cell is exposed to, the stiffness of the structure that the cell is placed in, and the mechanical forces, such as stretching, that it undergoes.

It’s this last factor that Schiele’s research team is honing in on.

“By applying mechanical stretches, we think we can differentiate these stem cells toward tendon,” Schiele said. “But we don’t really understand how it’s happening, or how these mechanical forces influence cell behavior, so we’re really trying to get at the processes behind that.”

The experiment of better understanding the process begins with stem cells harvested from mice, which provide a model system to represent adult human stem cells. Schiele’s students seed the cells into a small sponge, or scaffold, made of bovine collagen. Since this collagen protein is a major component of human tendon tissue, it acts as a natural mimic. It’s this type of material that could be sutured into patients’ torn or ruptured tendons to facilitate re-growth.

Once in the scaffold, the cells attach to the surface and spread out. Biological engineering sophomore Sophia Bowen of Sandpoint, who has worked in the lab since spring 2016, is experimenting with how many stem cells to place on each scaffold and if cell seeding density influences tendon formation.

Upon settling on an appropriate number of cells, students place the scaffolds in a custom device called a mechanical bioreactor system, which allows them to test how mechanical forces, like stretching, influence the behavior of the stem cells and the probability of differentiating toward tendon.

The bioreactor was designed and constructed in-house by biological engineering senior Abby Raveling of Hamilton, Mont. Raveling began with a template and then used the computer program SolidWorks to customize the device to suit the team’s needs.

The finished product has three chambers that each hold one collagen scaffold, held in place by grips. It also has three motors, which are operated through the code that Raveling and biological engineering graduate student Hee Jun Um, from South Korea, wrote in LabView programming language. Once turned on, the motors attach to the grips, move up and down, and stretch the scaffolds back and forth.

Students can apply various stretching parameters to the scaffolds, but they’ve maintained that the strain should be cyclical, or repetitive, rather than static.

“The reason we do it cyclically is to mimic the normal physiological environment,” Raveling said. “Because if you’re walking, your Achilles tendon is being cyclically strained as you walk.”

According to Sophia Theodossiou, a doctoral student in biological engineering from Athens, Greece, “tendon development seems to depend quite a bit on the mechanical loading that they experience.” Theodossiou began working in the lab during the summer of 2016.

After a given period of time of mechanical stretches, students remove the scaffolds from the bioreactor and stain them with a fluorescent dye to identify how the cells have been affected by the force — a process that Bowen, a former art major, finds particularly fascinating.

“There have been a lot of times when I’ve been looking at something in the lab and I’ve thought, ‘Wow, I could see this in an art gallery,’” she said.

In a preliminary experiment, the proteins in the stem cells with the highest percentage of stretch were elongated — an indication that the cells may turn toward tendon, Schiele said. And there seemed to be more intercellular connections.

“They were talking to each other and attaching better to our scaffolds,” Raveling said.

Theodossiou, who is studying how communication and the transmission of cells’ signals influences what lineage they travel down, is especially interested in this result.

“What is that magic discussion cells have when they’re turning into tendon versus fat or bone?” she said. “One of our hypotheses is that you have to have the right ratio of all these different communication proteins during development for the cells to turn into functional tendon. So if we know what the correct ratio of proteins is, and how mechanical stimulation affects them, then we can manipulate them to develop functional tendon in the lab.”

And, Schiele added, “Maybe we find that we need to turn on a specific cell signaling pathway. Or we need to turn on a specific cell behavior to better direct stem cell fate toward tendon.”

In July, Bowen presented her findings on cell seeding density to the Idaho Conference on Undergraduate Research.

In June, Raveling shared her findings from the bioreactor at the SB3C, or the Summer Biomechanics, Bioengineering and Biotransport Conference, in Tucson, Ariz. She was one of the only undergraduate students to conduct a podium presentation.

“The biological engineering department as a whole really encourages students to get involved in undergraduate research as freshmen,” Schiele said. “It’s a great hands-on learning opportunity where you can apply the skills you learn in your classes to real world problems. A student can be very productive and hopefully get abstracts and papers as an undergraduate student if they start their freshman year.”

Raveling and Theodossiou are currently working on a paper related to the bioreactor and plan to include open-source drawings of the device.

In the meantime, the team will continue conducting experiments to reach their final goal.

“If we can have a tendon treatment option that replaces diseased or damaged tissue and improves strength after healing,” Schiele said, “that would be a big benefit” — and possibly the end to devastating tendon injuries.